1. Field of the Invention
This invention relates to a new and improved method of making extremely accurate photoresist lift-off patterns having a deep undercut for use in the manufacture of Josephson junction superconducting devices. More particularly, this invention relates to an improved lift-off pattern which may be employed during isotropic vacuum deposition of low temperature superconducting metals such as lead, lead alloys of gold, indium and bismuth.
2. Description of the Prior Art
Additive metalization techniques, also known as "lift-off" techniques which employ lift-off patterns and lift-off masks were developed in the late 1960's. Such techniques were initially developed for use in vapor deposition of metals used in the manufacture of integrated circuit semiconductor devices. This prior art is largely classified in Class 427, subclasses 38, 88-91, 99 and 259 and also in Class 340, subclass 11. The book "Integrated Circuit Fabrication Techniques", by David J. Elliot, Copyright 1982 McGraw Hill, New York at pages 27 to 32 includes a summary of "metal lift-off" techniques.
Early multilayer lift-off pattern systems were designed for use in processes where the processing temperature exceeded 100.degree. C. Processes which are employed in the manufacture of lead-base Josephson junction superconducting devices such as lead, lead alloys of gold, bismuth and indium have very thin oxide junctions which can be easily damaged when process temperatures exceeds 70.degree. C.
U.S. Pat. No. 4,204,009 which is assigned to IBM, teaches a two layer photoresist lift-off system employing two separate layers of positive photoresist material in which a lift-off pattern is provided at low processing temperatures. The first positive layer of photoresist material is very thick and is unsaturated. The second positive photoresist layer is comparatively thin and is completely saturated. When the two layers are exposed as a laminate layer at the same time through a mask, the areas of both photoresist layers not covered by the mask are exposed to the collimated light source. When the exposed areas are developed, they are removed. Since the collimated light source is not perfectly polarized in a single axis, and the more dense and saturated upper layer of photoresist is less sensitive to the light source, there is more isotropic or edge diffusion of the light into the edges of the bottom layer when both layers are exposed as a laminar structure. Further, the edges of the bottom layer are more susceptable to light and are exposed to a greater degree. When the laminar structure is then developed, the bottom layer develops faster than the top layer and provides an undercut profile under the top layer in the same manner as would occur when chemically etching dissimilar metals. This IBM patent recognizes the temperature problems involved in the manufacture of Josephson junction superconductive devices and provides a low temperature process solution. However, the saturated photoresist which is employed must be specially mixed and specially filtered at the time of use. Further, the saturated photoresist material is quite sensitive to ambient temperatures. For example, as the temperature increases, the photoresist becomes less saturated. If the temperature drops, precipitation may occur in the saturated photoresist material. It is known that saturated photoresist materials have very low viscosity and must be applied in thin coatings. The process described in this IBM patent employs multiple thin coatings of saturated photoresist. When the top layer of photoresist material is made too thin, and a substantial undercut lift-off pattern is being made, the top layer will collapse. Further, since the bottom layer of the photoresist lift-off pattern is substantially thicker than the top layer, longer development time is required to achieve a desired undercut and this permits the developing solution to attack and develop the top thin layer at the edges thus destroying the accuracy of the pattern. The saturated photoresist will combine with and bleed into the unsaturated layer which changes the concentration of both layers at the interface boundary.
It would be desirable to provide a multilayer photoresist lift-off pattern which may be made at low processing temperatures and which maintains very high accuracy of the dimensions of the mask.